A new factor isolated from the rat submaxillary gland in combination with other hormones allows the growth of C6 rat glial cells, Balb 3T3 and Swiss 3T3 in a serum free medium in which the serum is replaced by mixtures of hormones. This new factor is distinct from NGF and EGF. It has a molecular weight of approximately 60,000 and an isoelectric point of about 4.5. Purification will continue until homogeneous preparations are obtained. Antisera against the purifird material will be prepared for the assay of this factor in rats under a variety of physiological conditions.